This invention relates to an electromechanical translation device which is called an INCHWORM TRANSLATOR by Burleigh Instruments, Inc., N.Y.
The electromechanical translation device is for axially moving a shaft with a controllable or incremental step of movement. The step of movement is typically of the order of a micron and may be less. Although each step is small, it is possible to move the shaft over a long distance and at a high speed. The distance may be 50 mm or more. The speed may be 2 mm/sec. The electromechanical translation device is capable of carrying out high resolution positioning, precision measurement, and intelligent electronic control either individually or in combination in, for example, an optical device. Such electromechanical translation devices are disclosed in the specifications of U.S. Pat. No. 3,902,084 issued to William G. May, Jr., and U.S. Pat. No. 3,902,085 issued to Richard A. Bizzigotti, both assigned to the above-mentioned Burleigh Instruments.
As will later be described more in detail, an electromechanical translation device comprises a shaft, a housing defining a hollow space in which the shaft is housed and is extended outwardly of the housing, an electromechanical driver fixed to the housing in the hollow space and having a first and second end surface and a cylindrical driver inner peripheral surface along which the shaft is freely movable, and a first and a second controllable electromechanical gripper or clamper attached in the hollow space to the first and the second end surfaces, respectively. Each of the first and the second grippers has a cylindrical gripper inner peripheral surface along which the shaft is controllably slidably movable. More particularly, each gripper is controllable so as to grip or clamp the shaft and to allow the shaft slidably move along its inner peripheral surface. A driving circuit comprises a power source and switches and is connected to the driver and the first and the second grippers for controllably supplying electric power thereto so as to make the driver and the grippers cooperate in axially stepwise or incrementally moving the shaft as will presently be described more in detail. The driver and the first and the second grippers may be three parts of a single continuous solid piezoelectric or electrostrictive member although it is preferred to individually manufacture the three parts and then assemble the parts into a unitary assembly. The switches preferably comprise switching transistors.
In a conventional electromechanical translation device, a single cylindrical electrostrictive member is used as each of the driver and the first and the second grippers. The electrostrictive member has an outer peripheral surface and comprises a pair of electrodes along the inner and the outer peripheral surfaces, respectively. Responsive to the electric power, each gripper radially contracts and expands to grip and release the shaft, respectively. When the driver axially expands in response to the electric power while the first and the second grippers release and grip the shaft, respectively, the shaft moves in a direction and sense from the second gripper to the first gripper. The first and the second grippers are subsequently caused to grip and release the shaft, respectively, while the driver is left expanded. When the driver thereafter axially contracts, the shaft further moves in the above-defined direction and sense. When the first and the second grippers are subjected to a reversed sequence of operation, the shaft stepwise moves in a reversed direction and sense. A combination of the shaft, the driver, and the first and the second grippers is operable in this manner like an inchworm. It is possible to use an electronic digital computer in controlling supply of the electric power and thereby to move the shaft at a high speed as described before. The computer may be a compact one that is known as a personal computer. At any rate, transverse electrostriction effect or piezoelectric unstiffened mode is used in the driver while longitudinal electrostriction effect of piezoelectric stiffened mode is used in each of the first and the second grippers.
It is well known in the art that the transverse electrostriction effect is weaker than the longitudinal one and is consequently disadvantageous in view of efficiency of the electric power. More particularly, a strain appears as elongation or contraction in an electrostrictive member approximately in linear proportion to the intensity of an electric field produced therein by the electric power. If the electric field has a predetermined intensity, the strain is small when the transverse electrostriction effect is used instead of the longitudinal one. On the other hand, a pair of electrodes must be formed in this event along the first and the second end surfaces, respectively. The driver has typically an inner diameter of 11 mm, a wall thickness of 1 mm, and an axial length of 25.4 mm. More than twenty times as high an electric voltage must therefore be supplied between the electrodes on producing the electric field of the predetermined intensity. The transverse electrostriction effect is used in the driver to avoid this high electric voltage despite the disadvantage in the efficiency. In practice, an expansion or contraction of about 1 micron is attained with application of an appreciably high electric voltage of 600 volts between the electrodes formed on the inner and the outer peripheral surfaces of the driver.
Although the longitudinal electrostriction effect is used in the first and the second grippers, the inner diameter varies only a few microns by application of the appreciably high electric voltage of 600 volts thereto. The shaft and the grippers must therefore be precisely machined in order to achieve free slide of the shaft relative to the gripper inner peripheral surface and also a predetermined gripping force. In fact, the shaft fits the grippers with nearly zero clearance when either of the grippers releases the shaft. This inevitably results in abrasion or wear of the electrode formed along each gripper inner peripheral surface.
The switching transistors must withstand the appreciably high electric voltage of, for example, 600 volts. Each switching transistor must therefore comprise an emitter and a collector electrode of a wide electrode area in order to prevent the current density from becoming excessive at the emitter and the collector electrodes. On carrying out the high-speed operation, the electric current flows only through the surface portion of each electrode due to the skin effect. In order to cope with the skin effect, each of the emitter and the collector electrodes must have a wider electrode area. It is true that a plurality of transistors of a low withstand voltage may be used in place of each switching transistor of a high withstand voltage so as to collectively withstand the appreciably high electric voltage. This, however, gives rise to a problem of delay. Furthermore it is troublesome to select low-withstand voltage transistors of uniform characteristics. In any event, the electromechanical translation device becomes bulky and expensive.